Jan-Mar 2000 Susceptibility of Sugarbeet Root Maggot 17 Susceptibility of Sugarbeet Root Maggot Tetanops myopaeforlnis (Diptera: Otitidae) Larvae to Terbufos, Chlorpyrifos and Aldicarb J. Scott Armstrong!, Robert J. Dregseth and Albin W. Anderson2 North Dakota State University, Department ofEntomology, Fargo, ND 58105 ABSTRACT Organophosphate and carbamate insecticides have been used for control of the sugarbeet root maggot, Tetanops myopaeformis (von Roder) in sugarbeet Beta vulgaris (L.) for over thirty years in the Red River Valley of North Dakota and Minnesota. Insecticides applied in furrow, at­ planting, have not always prevented significant larval feed­ ing injury in production sugarbeet fields. The reasons for inadequate granular insecticide performance have never been clear. This study determined lethal concentration values (LC & LC ) expressed in ppm for larval sugarbeet so 90 root maggots exposed to technical terbufos, chlorpyrifos and aldicarb. Mortality assays were conducted on larvae produced from adults collected from three locations in the Red River Valley from 1987 to 1997. Data combined across all years resulted in LCso values of0.023 for terbufos, 0.121 for chlorpyrifos, and 0.138 for aldicarb. LC90 values for all years combined were 0.051 for terbufos, 0.235 for chlorpyrifos, and 0.286 for aldicarb. The consistency of LCso and LC90 values from assays conducted over a ten year period strongly suggests that insecticide resistance is not a factor when the performance of granular terbufos, chlorpyrifos or aldicarb is inadequate. lCurrent address: Department of Plant and Soil Science, Texas Tech University, Box 42122, Lubbock TX, 79409. 2 Retired Professor Emeritus, North Dakota State University, Department of Entomology. Jan-Mar 2000 Susceptibility of Sugarbeet Root Maggot 19 In the two most recent years ofefficacy trials with granular insec­ ticides in the Red River Valley, aldicarb-treated test plots have resulted in some ofthe lowest mean damage ratings (Armstrong et a!., 1997, Annstrong et a\., 1998) achieved over the last twenty years. These low damage ratings occurred in the face of heavy adult fly activity as indicated by adult fly traps, and could be a result of lower usage of carbamate insecticides and thiocarbamate herbicides, which eventually lowered soil microbes respon­ sible for rapid degradation. Several areas in the Red River Valley experienced significant SBRM injury following the application of aldicarb at-planting in the early to mid 1980s. Rapid degradation ofcarbamate insecticides (Suett and Jukes 1988, Read 1987) applied to soil was not fully understood at the time con­ trol failures were occurring. However, control failures were widely be­ lieved to be the result of insecticide resistance of the SBRM to carbamate insecticides. Organophosphate insecticides will oxidize at significantly dif­ ferent rates dependant upon previous insecticide use (Read 1987), soil type and pH (Laveglia & Dahm, 1975), suggesting that accelerated degra­ dation could contribute to the failure of insecticides to control SBRM.. The following study was undertaken to establish lethal concentra­ ) tion values (LClO LC9o for larval SBRM collected over a ten year period. The values could·then be used as base-line data, to determine if the sllscep­ tibility of the SBRM exposed to organophosphate, carbamate and terbufos changed over time, and if changes could be recognized as true insecticide resistance. SBRM larvae used in this study were collected from fields with high densities. Some of the larvae collected were from fields where control failures were reported for carbamate insecticides. MATERIALS AND METHODS Neonate, first instar SBRM larvae were used to determine lethal ) concentration (LClO and LC9o values. Larvae used for testing were pro­ duced from collecting third instar larvae from the soil of producers fields near St. Thomas, ND, Hillsboro, ND and Glyndon, MN starting in the late SUITUl1ers of 1986 to 1997. The larvae collected in 1986 were not tested until 1987 because of time requirements of the rearing process. SBRM larvae enter a slow transition into diapause in late summer and require 6 months at 38F to complete diapause and pupation. The SUlTUTler collected larvae were removed from cold storage and left at room temperature in petri dishes for pupation and adult emergence. Adult males and females were paired and placed in adult cages for mating. Eggs were collected from black cloth provided at the bottom of the adult cages. Twenty to twenty five neonate larvae were placed on 4.6 cm diameter circular paper-towel disks 20 10urnal of Sugar Beet Research Vol 37, No I (Fort Howard Corporation, Green Bay, WI) and treated with dilutions of technical grade aldicarb (Temik®, Rhone-Poulenc, Alexander Park, NC), chlorpyrifos (Lorsban®, Dow AgroSciences, Indianapolis, IN) and terbufos (Counter®, American Cyanamid, Princeton, NJ) in acetone. Treatments were replicated five times for a total of 100 to 125 treated larvae and 25 non­ treated larvae were used as controls. Control larvae were placed on paper­ towel disks treated with acetone. The acetone was allowed to dissipate for one hour before placing any of the treatment groups, including the controls on paper-towel disks. Aldicarb and chlorpyrifos dilutions were 0.06, 0.13, 0.17,0.21,0.31 ppm. Terbufos dilutions were 0.01 , 0.02, 0.03, 0.05, and 0.06 ppm. Mortality was assessed at 3 hr of exposure. Neonate larvae of the SBRM are transparent where systolic movement ofthe heart can clearly be seen. Larvae that had no movement of the heart were considered dead. A control group was used for each collection site tested; however, a truly susceptible population could not be tested because SBRM can not be reared in the laboratory from egg to adult, preventing the utilization of a known population that has never been exposed to insecticide. The data were analyzed with probit analysis (LeOra Software, 1987) where LC50 and LC90 values were calculated with 95% confidence intervals. A composite curve that included all mortality data for all years was calculated for future resistance-ratio reference. RESULTS AND DISCUSSION Terbufos had the lowest LC and LC values indicating that it is so 90 more toxic to SBRM than chlorpyrifos or aldicarb in a petri dish assay. The overall LCso for all years tested was 0.023 ppm while the LC90 was 0.051 ppm (Table 1). The consistency of upper and lower 95% confidence limits indicates that the individuals collected and tested at St. Thomas and Glyndon did not show signs of insecticide tolerance or resistance, although a sus­ ceptible population was not used for comparative purposes and resistance­ ratios could not be compared. Neither the St Thomas nor Glyndon sugarbeet producing areas had significant control failures with terbufos. Mortality assays using technical chlorpyrifos resulted in an LCso value of 0.121 and a LC90 value of 0.235 ppm when all years were com­ bined (Table 2). Aldicarb was similar to chlorpyrifos with LC and LC so 90 values of 0.138 and 0.286 ppm, respectively (Table 3). All three locations (St Thomas, ND, Hillsboro, ND, and Glyndon, MN) experienced control failures following the at-planting use of aldicarb in the mid 1980s. The individuals tested in 1987 were collected from a field that sustained sig­ nificant injury after aldicarb was used at-planting in 1986. The progeny of these individuals showed no sign of resistance. The most probable cause 20 Journal of Sugar Beet Research Vol 37, No I (Fort Howard Corporation, Green Bay, WI) and treated with dilutions of technical grade aldicarb (Temik®, Rhone-Poulenc, Alexander Park, NC), chlorpyrifos (Lorsban®, Dow AgroSciences, Indianapolis, IN) and terbufos (Counter®, American Cyanamid, Princeton, NJ) in acetone. Treatments were replicated five times for a total of 100 to 125 treated larvae and 25 non­ treated larvae were used as controls. Control larvae were placed on paper­ towel disks treated with acetone. The acetone was allowed to dissipate for one hour before placing any of the treatment groups, including the controls on paper-towel disks. Aldicarb and chlorpyrifos dilutions were 0.06, 0.13, 0.17,0.21,0.31 ppm. Terbufos dilutions were 0.01,0.02, 0.03,0.05, and 0.06 ppm. Mortality was assessed at 3 hr of exposure. Neonate larvae of the SBRM are transparent where systolic movement ofthe heart can clearly be seen. Larvae that had no movement of the heart were considered dead. A control group was used for each collection site tested; however, a truly susceptible population could not be tested because SBRM can not be reared in the laboratory from egg to adult, preventing the utilization of a known population that has never been exposed to insecticide. The data were analyzed with probit analysis (LeOra Software, 1987) where LC and LC values were calculated with 95% confidence 50 90 intervals. A composite curve that included all mortality data for all years was calculated for future resistance-ratio reference. RESULTS AND DISCUSSION Terbufos had the lowest LC50 and LC90 values indicating that it is more toxic to SBRM than chlorpyrifos or aldicarb in a petri dish assay. The overall LC50 for all years tested was 0.023 ppm while the LC90 was 0.051 ppm (Table I). The consistency of upper and lower 95% confidence limits indicates that the individuals collected and tested at St. Thomas and Glyndon did not show signs of insecticide tolerance or resistance, although a sus­ ceptible popUlation was not used for comparative purposes and resistance­ ratios could not be compared. Neither the St Thomas nor Glyndon sugarbeet producing areas had significant control failures with terbufos. Mortality assays using technical chlorpyrifos resulted in an LCso value of 0.121 and a LC90 value of 0.235 ppm when all years were com­ bined (Table 2). Aldicarb was similar to chlorpyrifos with LC;o and LC90 values of 0.138 and 0.286 ppm, respectively (Table 3).